Polychlorinated biphenyls (PCBs), a group of industrial oils with two linked phenyl rings and various degrees of chlorination, were used widely between the 1930s and the 1970s, when they were banned in much of the world. They are still among the most ubiquitous and persistent of environmental contaminants, and are detectable in most human beings worldwide. PCBs per se are toxic, teratogenic, and can cause impaired learning and behavioral abnormalities in laboratory animals. In children, transplacental exposure even at low, background levels may produce subtle psychomotor, cognitive, and memory impairment. The most severe effects, however, from exposure to these compounds in children occurred among the offspring of women poisoned by complex mixtures of heat-degraded PCBs. Two such exposures have occurred, one in Japan (Yusho) in 1968 and the other in Taiwan (Yucheng) in 1979 (the words mean ?oil disease? in Japanese and Chinese respectively). PCBs used as heat exchangers in the processing of rice bran cooking oil leaked into the finished oil. The PCBs themselves were partially heat-degraded and thus contaminated by polychlorinated dibenzofurans (PCDFs) and other chlorinated, multi-ring compounds. Some of the PCDFs are extraordinarily toxic compounds, with potencies approaching that of 2,3,7,8 tetrachlorodibenzo-p-dioxin, a structurally similar compound that is the most toxic synthetic chemical known. PCBs and PCDFs are not well excreted, and the women continued to have affected children for years after the exposure occurred. In Japan, the children born to affected women were ?hypotonic, apathetic, and dull? as 9 and 10 year olds, but there has been no formal study of them. In Taiwan (the Yucheng incident), over 2,000 people consumed the contaminated oil for about 6 months during 1978-9. In the first three years after the outbreak, eight of the 39 children born to exposed women died. In 1985, we studied 117 of the 123 children with transplacental exposure (Yucheng children), and found ectodermal defects, developmental delay, more behavioral problems and higher activity levels. These children have been followed since 1985 with cognitive and behavioral testing. Between 1985 and 1992, the Yucheng children's cognitive test scores were not improving relative to controls as they got older, nor was the effect smaller in children who were born up to 6 years after the exposure had taken place. From 1985 until 1992, the children's behavior was studied with translations of the Rutter Child Behavior Scale A, which measures problem behavior, and the Werry-Weiss-Peters scales of children's activity. The Yucheng children had scores about 23% higher on the Rutter and about 15% higher on the Werry-Weiss-Peters scale. As with the cognitive testing, the difference in scores between the Yucheng and control children did not diminish as the children aged nor by year of birth. Continued follow-up cognitive testing showed some diminution of effect in the oldest children studied with Raven's Standard Progressive Matrices but results of IQ and behavioral testing have not yet been reported. We presented data from follow-up behavioral and cognitive testing of the same children using the Achenbach Child Behavior Checklist (CBCL), the Rutter Child Behavior Scale A, and the Wechsler Intelligence Scales for Children-Revised (WISC-R). Aims: In 1978, about 2000 persons in Taiwan were poisoned when their cooking oil was contaminated during manufacture with heat-degraded polychlorinated biphenyls, which are toxic, very widespread pollutant chemicals. The chemicals cannot be metabolized or excreted, and 8 of the first 39 children born to affected women died. When examined in 1985, the 117 surviving children were found to have ectodermal defects, developmental delay, and disordered behavior. We have continued to follow the children. Accomplishments: The exposed children scored 3 points (p = .05) lower than controls for IQ; 3 points (p = .002) higher on the CBCL (an effect size similar to the gender difference) and 6 points (p < .001) higher on the Rutter scale (3 times the gender difference). Birth year by exposure interactions, testing whether children born long after the exposure were as affected as those born soon after, were small and not significant. Age by exposure interactions, testing whether the children improved relative to controls as they got older, were significant only for the Rutter scale. Prenatal exposure to these compounds produces long lasting cognitive and behavioral damage, but there is some evidence for recovery.